162 CAPILLARY CIRCULATION. [BOOK i. 



with the heart's beat, are frequently visible ; but these disappear 

 in the capillaries, in which the flow is even ; that is, not broken by 

 pulsations, and this evenness of flow is continued on along the 

 veins so far as we can trace them. Not infrequently variations in. 

 velocity and in the distribution of the blood, due to causes which 

 will be hereafter discussed, are witnessed from time to time. 



The character of the flow through the smaller capillaries is 

 very variable. Sometimes the corpuscles are seen passing through 

 the channel in single file with great regularity ; at other times 

 they may be few and far between. Some of the capillaries, as 

 we have said, are wide enough to permit two or more corpuscles 

 abreast. In all cases the blood, as it passes through the capillary, 

 stretches the walls and expands the tube. Sometimes a corpuscle 

 may remain stationary at the entrance into a capillary, the channel 

 itself being for some little distance entirely free from corpuscles. 

 Sometimes many corpuscles will appear to remain stationary in one 

 or more capillaries for a brief period, and then move on again. Any 

 one of these conditions readily passes into another ; and, especially 

 with a somewhat feeble circulation, instances of all of them may 

 be seen in the same field of the microscope. It is only when the 

 vessels of the web are unusually full of blood that all the capil- 

 laries can be seen equally filled with corpuscles. The long, oval,, 

 red corpuscle moves with its long axis parallel to the stream, 

 occasionally rotating on its long axis, and sometimes, in the larger 

 channels, on its short axis. The flexibility and elasticity of a 

 corpuscle are well seen when it is being driven into a capillary 

 narrower than itself, or when it becomes temporarily lodged at 

 the angle between two diverging channels. 



These, and other phenomena on which we shall dwell later on, 

 may be readily seen in the web of the frog's foot or in the 

 stretched-out tongue or in the mesentery of the frog ; and essen- 

 tially similar phenomena may be observed in the mesentery or 

 other transparent tissue of a mammal. All over the body, 

 wherever capillaries are present, the corpuscles and the plasma 

 are being driven in a continuous, and though somewhat irre- 

 gular, yet, on the whole, steady flow through channels so minute 

 that the passage is manifestly attended with considerable diffi- 

 culties. 



It is obvious that the peculiar characters of the flow through 

 the minute arteries, capillaries, and veins, afford an explanation 

 of the great change, taking place in the peripheral region, between 

 the arterial flow and the venous flow. The united sectional area 

 of the capillaries is, as we have seen, some hundreds of times 

 greater than the sectional area of the aorta ; but this united 

 sectional area is made up of thousands of minute passages, vary- 

 ing in man from 5 to 20 JJL, some of them, therefore, being in 

 an undistended condition, smaller than the diameter of a red 

 corpuscle. Even were the blood a simple liquid free from all 



